This invention relates to the use of seismic data, and more particularly the use of amplitude versus offset, AVO, analysis of seismic data, to generate direct indicators of the presence of hydrocarbons in subsurface earth formations.
In conventional seismic prospecting, acoustic energy is applied to the earth's surface. As the energy travels downward, it is reflected from subsurface interfaces back to the earth's surface. The amplitude of the reflected energy is normally recorded in the form of a series of time samples. By plotting these amplitudes versus a time scale, a representation of the locations and shapes of the subsurface interfaces is generated. The depths of the various interfaces correspond generally to the time of arrival of the various signals.
Seismic prospecting has been improved by techniques such as the common depth point, CDP, or common midpoint, CMP, techniques to improve signal to noise ratio. In these methods, seismic signals are generated sequentially at each of a number of points along the seismic prospecting path while reflections are recorded at all the points following the generation of each signal. The recorded signals are then organized into gathers of traces, each corresponding to a common depth point or common midpoint. That is, all of the traces in the gather result from source receiver pairs equally spaced about the point in question along the prospect path. By properly combining all of the signals corresponding to a CDP, the signal to noise ratio is improved.
Various attempts have been made to analyze CDP data in order to estimate various characteristics of the subsurface formations which may be used to predict the hydrocarbon bearing potential thereof. One important method is known as the amplitude versus offset, AVO, technique in which the variation in amplitude of signals reflected from a given subsurface interface is analyzed for changes relating to the angle of incidence or offset between source receiver pairs. It is generally believed that the presence of hydrocarbons in a subsurface formation causes an increase in the change in amplitude as a result of offset. Improvements on the AVO techniques have resulted in the generation of what are believed to be direct indicators of the presence of hydrocarbons in subsurface formations. However, testing of these methods in areas with known hydrocarbon bearing layers has shown that current methods often fail to identify important hydrocarbon bearing zones.